1. Field of the Invention
The present invention relates to a process for refining vegetable oil by subjecting a miscella obtained by extraction of vegetable seeds with an organic solvent to ultrafiltration.
2. Discussion of Background
Conventional vegetable oils used as edible oils include soybean oil, rapeseed oil, safflower oil, corn oil, sunflower oil and rice bran oil. To obtain such vegetable oils, it is common to extract a raw material with an organic solvent such as n-hexane to obtain a miscella and to remove the organic solvent from this miscella to obtain a crude glyceride oil. This crude glyceride oil usually contains from 0.5 to 3% by weight of waxes such as higher alcohols and impurities such as peptides, organic sulfur compounds, carbohydrates, fatty acids, hydrocarbons, sterol, lower ketones, lower aldehydes and coloring matters, in addition to phospholipids such as lecithin.
It is necessary to remove such impurities from the crude glyceride oil, since such impurities tend to decompose or polymerize, whereby oils tend to be colored and bad odors are likely to be generated. As a conventional process for removing impurities from a crude glyceride oil, there is, for example, a process as shown in FIG. 3, wherein firstly, in a process of removing gums, water is added to the crude glyceride oil to hydrate the gum substance composed mainly of phospholipids (hydration), followed by centrifugal separation to remove the gum substance. Even by this hydration, phospholipids can not completely be removed. Therefore, at the beginning of the subsequent process of removing acids, phospholic acid (or citric acid) is introduced to further form a gum substance, and then an alkali such as caustic soda is added for saponification of fatty acids, so that the gum substance and fatty acids form a slime, and the slime is then removed by a centrifugal separator. Further, water is added for the purpose of e.g. removal of the added reagents to form a slime, and this slime is removed by a centrifugal separator. Then, in a bleaching and deodorizing step, coloring matters and impurities which could not be removed by the above alkali refining, such as heavy metals, fatty acids, soap materials, and gum substance, are removed by adsorption and vacuum distillation to obtain purified vegetable oil. As described in the foregoing, in the conventional process for removing the gum substance, slime, etc. from the crude glyceride oil, the purification process is complicated, the utility costs such as costs for chemical reagents are substantial, and the vegetable oil is lost when the gum substance, etc. are discharged from the centrifugal separator.
In a process proposed to solve such problems, Japanese Unexamined Patent Publication No. 153010/1975 proposes to use an ultrafiltration membrane made of polysulfone, polyaclylonitrile or polyamide and Japanese Unexamined Patent Publication No. 19499/1983 proposes to use a polyimide-type ultrafiltration membrane, and the proposed process comprises diluting a crude glyceride oil with an organic solvent such as n-hexane, contacting the diluted oil with the ultrafiltration membrane under pressure, and removing the organic solvent from the permeate to obtain purified vegetable oil.
However, in the process wherein the above-mentioned ultrafiltration membrane is employed, a non-aqueous organic solvent is contacted with an organic compoundbased filtration membrane, whereby the operation temperature is rather limited, and it is required to cool the miscella obtained from the extraction process. Further, when used for a long period of time, the membrane tends to undergo swelling by the influence of the organic solvent in the miscella, whereby there will be a problem such that the degree of selectivity of cut off limit tends to deteriorate.